1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, such as a semiconductor integrated circuit, and more particularly to a method for forming isolation regions to delimit active regions on a semiconductor substrate.
2. Prior Art
To electrically isolate the active regions, where semiconductor devices, such as transistors, are formed, on a silicon semiconductor substrate for example, isolation regions made of silicon oxide are generally formed, and semiconductor devices are formed as required in the active regions delimited by the isolation regions.
One of the above-mentioned methods for forming an isolation region is the LOCOS isolation process. According to this process, the surface of a silicon semiconductor substrate is partially covered with an oxidation inhibitor film formed by a silicon nitride film, for example, which has been patterned, and the regions of the surface of the semiconductor substrate which are exposed through the openings of the oxidation inhibitor film can be oxidized locally by thermal oxidation. Therefore, isolation regions are formed by a field oxide film made of silicon oxide generated by the oxidation process mentioned above.
Incidentally, as an improved type of the LOCOS process, there is the FLOCOS (Flamed LOCOS) process. According to this proior improved technique, side-walls are provided at the edge portions of a patterned opening in an oxidation inhibitor film to enable patterning of finer patterns in an oxidation inhibitor film than specified in the design rule applicable to patterning of oxidation inhibitor films, so that finer isolation regions can be obtained than are specified in the pattern design rule. Moreover, it becomes possible to inhibit the so-called bird""s beak of the isolation region from encroaching into the active region.
However, in the conventional FLOCOS isolation process in which side-walls are provided for the oxidation inhibitor film, even if heat treatment is carried out on the semiconductor substrate to oxidize the region of the surface of the semiconductor substrate which is exposed through the opening of the oxidation inhibitor film, it often happens that the exposed region is not oxidized and therefore an oxide film for use as the isolation region cannot be grown sufficiently. Such a situation is often encountered when isolation regions made of a field oxide film are to be located less densely and secluded from one another.
The object of the present invention is to provide an isolation-region forming method by which isolation regions can be formed with a higher yield than before even when the isolation regions are formed sparsely.
The present invention derived from a basic concept that effects by the deposit, which builds up on the exposed region, should be precluded from heat treatment of the exposed region to generate a field oxide film. This concept was developed from a viewpoint as follows: an insufficient growth of the field oxide film on the exposed region of the surface of the semiconductor substrate, revealed through an opening of the oxidation inhibitor film, in heat treatment of the semiconductor substrate is caused by part of the material for the oxidation inhibitor film building up on the exposed region when the unwanted side-wall material is removed in the formation of the side-walls on the oxidation inhibitor film by the plasma etching process prior to the heat treatment.
According to an aspect of the present invention, there is provided a method for forming an isolation region made of a field oxide film generated by thermal oxidation of an exposed region of the surface of a semiconductor substrate partially covered by an oxidation inhibitor film, comprises partially covering the surface of the semiconductor substrate with an oxidation inhibitor film; depositing a side-wall material on the oxidation inhibitor film and on an exposed region of the surface, revealed through an opening of the oxidation inhibitor film, to form side-wall parts at edge portions of the oxidation inhibitor film by a material having the same function as the oxidation inhibitor film; then, removing by a plasma etching process, unnecessary portions of the side-wall material deposited on the oxidation inhibitor film and on the exposed region of the semiconductor substrate and leaving intact the side-wall parts at the edge portions of the oxidation inhibitor film; and carrying out a cleaning process on the exposed region of the surface of the semiconductor substrate, revealed through an opening of the oxidation inhibitor film, before performing subsequent heat treatment to generate a field oxide film.
According to a second aspect of the present invention, after unnecessary portions of the side-wall material deposited on the oxidation inhibitor film and on the exposed region of the semiconductor substrate are removed, leaving intact the side-wall parts at the edge portions of the oxidation inhibitor film, a cleaning process by an etching process is carried out on the exposed region of the surface of the semiconductor substrate, revealed through an opening of the oxidation inhibitor film, before subsequent heat treatment is performed to generate a field oxide film.
According to a third aspect of the present invention, even if part of the material for the oxidation inhibitor film builds up on the exposed region when the plasma etching process is performed to form the side walls, after removal of the unnecessary side-wall material by the plasma etching process, by the cleaning process performed on the exposed region on the surface of the semiconductor substrate prior to the heat treatment, the deposit may be removed securely from the exposed region, and by the heat treatment on the cleaned semiconductor substrate, an isolation region made of a field oxide film may be formed on the exposed region with a high yield.
According to a fourth aspect of the present invention, as an etching process to be used for removal of the deposit, it is possible to adopt a dry etching process chiefly using radicals or a wet etching process using an etchant. By an etching process such as these, the deposit may be removed securely without having particles of the material of the deposit re-accumulate on the exposed region.
According to a fifth aspect of the present invention, before depositing a material for the side-wall parts, covering the exposed region on the surface of the semiconductor may be covered by an oxide film and after the unnecessary portions of the side-wall material are removed, the oxide film covering the exposed region may be removed by carrying out the cleaning process on the exposed region. Thus, the deposit from the oxidation inhibitor film that accumulates on the oxide film may be removed together with the oxide film.
According to a sixth aspect of the present invention, another method for forming an isolation region made of a field oxide film generated by thermal oxidation of an exposed region of the surface of a semiconductor substrate partially covered by an oxidation inhibitor film, comprises partially covering the surface of the semiconductor with an oxidation inhibitor film, the surface of the oxidation film being covered by a protective film; depositing a material for side-wall parts on the oxidation inhibitor film and on an exposed region of the surface, revealed through an opening of the oxidation inhibitor film, to form side-wall parts on edge portions of the oxidation inhibitor film by a material having the same function as the oxidation inhibitor film; then, by a plasma etching process, removing unnecessary portions of the side-wall material deposited on the protective film and on the exposed region on the semiconductor surface and leaving intact the side-wall parts on the edge portions of the oxidation inhibitor film; and carrying out heat treatment on the semiconductor substrate to generate the field oxide film.
According to a seventh aspect of the present invention, in the one other method mentioned above, when the unnecessary side-wall material which is not required to form the side wall parts is removed, the protective film covering the oxidation inhibitor film protects the oxidation inhibitor film beneath the protective film from the plasma etching process carried out for the removal, with the result that part of the material for the oxidation inhibitor film is prevented from building up on the exposed region.
Therefore, in heat treatment to generate the field oxide film, the growth of the field oxide film on the exposed region on the surface of the semiconductor substrate is not limited by the deposit unlike in the prior art, so that the field oxide film can be grown securely and the isolation region made of this field oxide film can be grown with a high yield.